DYNAMICS OF THE TOBA MAGMATIC SYSTEM AS RECORDED BY CRYSTALS

Just 74,000 years ago, one of Earth’s largest volcanic eruptions evacuated \u3e2800 km3 of magma from an upper-crustal batholith underlying the Toba caldera complex (Indonesia). Crystal cargo in the rhyodacitic-rhyolitic ignimbrite, known as the Youngest Toba Tuff, is texturally and chemically diverse and dominated by ≤20 mm crystals and clusters of quartz, plagioclase, and sanidine. Examination of the textural-chemical stratigraphy of these major minerals reveals a previously unrecognized and unprecedented archive of rapid crystallization and offers a new perspective on the origins of chemical and physical gradients inside the magma reservoir. Quartz crystals are rife with disequilibrium textures including embayments, melt inclusions, spongy morphologies, hollow faces, subgrain boundaries, multiple growth centers, and Ti-enriched arborescent zoning. Combined results of qualitative and quantitative analyses (petrography, CL, EBSD, X-ray µCT, LA-ICP-MS) indicate that textures commonly thought to signify crystal resorption, deformation, synneusis, or fluctuating P–T conditions are here a consequence of rapid disequilibrium crystallization. A model is proposed whereby early skeletal to dendritic quartz growth creates a causal sequence of textures derived from lattice mistakes, which proliferate during subsequent stages of slower polyhedral growth. The same instabilities and defects form when slow polyhedral growth transitions to fast skeletal-dendritic growth. Such morphological transitions are traceable to strong degrees of undercooling and supersaturation. These findings imply that thermodynamic disequilibrium crystallization can account for primary textural and chemical heterogeneities in igneous quartz and will impact its application as a petrologic tool. Plagioclase exhibits disequilibrium textures including sieve and skeletal morphologies, reaction zones, and has anorthite contents ranging from An20-86. Sanidine displays early sector growth, dissolution-regrowth, plagioclase occlusion, and has restricted orthoclase contents (Or74.5-77). Plagioclase across the magma compositional range overlaps in trace elements, but sanidine is chemically unique per high- and low-silica rhyolite and absent from rhyodacite. Integration of CL imagery and mineral chemistry reveals feldspar and quartz signatures of cumulate remelting and rejuvenation as a consequence of mafic-intermediate recharge and magma mixing. Rapid crystallization appears related to both magma mixing and shallow storage and dynamics of high-silica rhyolite. These processes combined with extensive crystal fractionation led to a compositionally zoned magma body